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Light Soaking: NREL PV Module Reliability Workshop, February 2011 This presentation does not contain any confidential or proprietary information 1

Light Soaking Effects on PV Modules: Overview and Literature Review

Michael Gostein and Lawrence Dunn

Atonometrics, Inc. Austin, Texas 78757

www.atonometrics.com

Prepared for NREL PV Module Reliability Workshop February 16-17, 2011

http://www.atonometrics.com/


Abstract

Light exposure of PV modules can produce a variety of effects including reversible metastable phenomena which influence the accuracy of PV module power output determination and long-term phenomena which affect power output stability of installed modules.

We present a brief review of technical literature on the effects of light exposure on different PV module technologies, including

a-Si/c-Si, CdTe, CIGS, and c-Si, addressing: the physical mechanisms of light-induced changes in each PV technology; long-term light-induced degradation effects; and current literature knowledge on PV module preconditioning for accurate power output determination.

Our poster is intended to provide an overview and to promote discussion on these subjects amongst workshop attendees.


a-Si / c-Si


Amorphous Silicon

Typical device structure, Ref [1]A

Light-induced degradation (LID) causes ~10-30% efficiency loss in first several hundred hours of light soak, Ref. [2]B

IEC 61646 qualification test introduced extended duration light soak requirement for module stabilization o Light soak until power varies <2% in

successive 43 kW-hr/m2 periods

Microcrystalline silicon (c-Si) shows increasing level of LID depending on amorphous content, Refs. [3],[4]

From Ref. A

From Ref. B, page 511

30%

15%


Staebler-Wronski Effect in a-Si

Staebler-Wronski effect (SWE) o Staebler and Wronski, 1977, Ref. [5] o Reduction in dark conductivity and photoconductivity of a-Si:H after light exposure o Reversible by annealing >150 C

Mechanism, Ref. [6] o Recombination-induced breaking of weak Si-Si bonds by optically excited carriers

after thermalization, producing defect centers that lower carrier lifetime o Self-limiting effect

Details o Many proposals, but exact microscopic mechanism not fully understood o Intrinsic effect – does not depend on impurities, Ref. [7] o Occurs in bulk of material, with additional surface contribution, Ref. [6] o Defects introduced by e.g. current injection produce different results, Ref. [6],[8],[9] o Accelerated testing possible using high-intensity pulsed light, while maintaining

standard operating temperature, Ref. [8] o Annealing behavior correlated with H diffusion, Ref. [6],[7]

Review of possible defect structure models and reaction mechanisms, Ref. [9]

Recent theoretical analysis and proposed mechanisms, Ref. [7],[10]

Simulation of a-Si:H device performance upon light exposure, Ref. [11] o Thinner cells show reduced light-induced performance degradation


Seasonal Effects in a-Si

Seasonal effects o Correlation of a-Si performance with daily

mean temperature, Ref. [2]A o Due to partial annealing of defects causing

SWE o 10-15% relative changes

Temperature effects in light soaking o Stabilized efficiency depends on

temperature during exposure, Ref. [12]B o SWE degradation vs. annealing o Warm-soak = higher efficiency

Stabilization of module performance correlated with temperature at installation site, 2008 study o Modules installed at higher-temperature

locations have better performance, Ref. [13]

o Similar for single, dual, triple-junction

From Ref. A, page 512

From Ref. B. Cold soak ~ 25 °C; Warm soak ~ 50 °C.


CdTe


CdTe Cell Structure

CdTe device structure: TCO / n-CdS / p-CdTe / back-contact ([14]A, [15], [16], [17]B)

Back-contact metallization problematic – requires high work function for ohmic contact o Various back-contact metallizations used (see e.g. [18]). o A Te-rich interfacial layer is beneficial. Ref. [18], [19] o A Cu component is beneficial, although Cu diffusion causes stability issues.

From Ref. B

From Ref. A, page 209


Extended Duration Light Soaking of CdTe

Extended duration studies of light soaking reveal need for long-term testing. [20]A

Accurate determination of long-term performance required >5000 hours of light soaking per sample.

High temperature accelerates degradation [21]B

From Ref. A

From Ref. B


Copper Diffusion from Back-Contact in CdTe

Cu diffusion, Ref. [22]A,[23],[24]B o Back contact in CdTe forms diode of

opposite polarity, limiting performance See e.g. modeling in [25]

o Addition of Cu lowers back-barrier height and improves J-V performance [24]

Cu loss via diffusion through CdTe (e.g. at high temperature) increases back-barrier height and reduces fill factor

Light soaking stress leads to efficiency loss

From Ref. A

From Ref. A after B


Effect of Bias During Light-Soaking of CdTe

CdTe degradation during extended light soaking is strongly affected by bias condition Ref. [22]A, [21]B,[20]C

Degradation rate increases with increased temperature, Ref. [21]

Degradation significantly faster at 100C than in the field. Can use high temperature as accelerated test, Ref. [21]

From C

Efficiency drop during extended stress as function of bias condition. From Ref. A after B


Stability in Various CdTe Devices

Stability of CdTe modules, Ref. [26]A o Light soaking stress yielded initial efficiency

improvement, followed by degradation

Metastable effects in CdTe, Ref [27] o Measurements of I-V curves in-situ during

extended light soaking of CdTe modules o Shifts in Isc, Voc, and FF versus light exposure o Voc could move either up or down with

exposure, depending on fabrication details o I-V parameters depend on module stabilization

Investigation of CdTe back contacts, Ref [28] o Without Cu: initial performance similar to devices

with Cu following stress

Degradation in CdTe with Sb-based back contacts o Back contacts based on Sb2Te3/Mo yielded stable

cells, Ref. [18]B o Sb2Te3-based back contacts, Ref. [29]

Outdoor testing of CdTe for 1.5 years Degraded ~5% in Voc and 8-10% in FF

From Ref. A

Stability vs. back contact materials. From Ref. B


CIGS


CIGS Device Structure

CIGS = Copper Indium Gallium Selenide = Cu(In,Ga)Se2

Device structure, Ref. [30]A,[31], [18]

Typically uses CdS buffer layer

Typically formed in substrate configuration o If superstrate, undesirable

CdS diffusion during CIGS deposition

Desire for structures without CdS o Various approaches

From Ref. A


CIGS Light Soaking Effects

Reversible metastability of photoconductivity in CIGS films, Ref [32] o Annealing at 80 °C leads to decrease of dark conductivity by ~2x at room

temperature o By exposure to light, the initial state can be re-established

Voc of CIGS cells shifts reversibly with light exposure or bias, Ref [33] o CIGS Voc rises upon light exposure or forward bias in dark, with corresponding rise in

efficiency of ~5% o Time scale ranges from minutes to hours; faster at higher temperature

Effects of sweep rate and voltage bias on CIS cells, Ref [34]A o Light soaking produced 7-15%

improvement in cell efficiency o Light soaking effect more pronounced

and longer lived than effect of forward bias

From Ref. A


CIGS Metastability Mechanisms

Three effects in CIGS metastabilities, Ref [35],[36] o Light soaking with white light may produce overall beneficial effect due to balance of

beneficial and detrimental effects [36] o Red light: increase in Voc, due to increase in carriers in absorber

Similar effect from forward bias o Blue light: interface effect o Reverse bias: interface effect

Proposed mechanisms, Ref. [36] o Metastable defects that trap carriers o Reversible migration of Cu

Amphoteric Se-Cu divacancy (VSe-VCu) complex o First principles calculations, Ref. [36]

VSe-VCu complex can act as amphoteric (donor or acceptor) defect

Defect state converted by light absorption Explains observed effects of red light, blue

light, and reverse bias o Experimental support

Refs. [37], [38], [39], [40]A

From Ref. A


Light Soaking in CIGS with Alternative Buffer Layers

Light soaking effects vary greatly depending on the device structure and especially the buffer layer composition.

Strong light soaking effect observed: o Superstrate CGS/CIGS cells grown with ZnO buffer instead of CdS, Refs. [41], [42] o ZnO/CIGS cells without CdS, Ref. [43] o Zn1-xMgxO buffer layers without CdS, Ref. [44]

Minimal light soaking effect observed: o CIGS with ZIS (Zn-Indium-Se) buffer layer (alternative to CdS), Ref. [45]


Thin-Film Pre-Conditioning


Thin-Film Pre-Conditioning

Due to metastability phenomena, preconditioning is essential for accurate power output determination of thin film PV modules.

However, due to the complexity of the phenomena and variability between different module technologies, reliable preconditioning methods are difficult to establish.

Current standard for stabilization in thin-film PV modules is IEC 61646 o <2% change after successive 43 kW/m2 exposure periods o Designed primarily for a-Si where dominant degradation is via Staebler-Wronski effect

Questions: o Can modules be stabilized via dark soaking (at bias? at temp?), without light soaking? o What are optimal temperatures and durations for stabilization?

Recent CdTe preconditioning examples: o CdTe efficiency found to initially improve, then degrade, with light exposure, Ref. [26] o CdTe efficiency measurements depend strongly on pre-conditioning conditions, with

various conditions yielding higher or lower efficiency results, Ref. [46]

Recent NREL studies on pre-conditioning techniques for CdTe & CIGS, Refs. [47], [48], [49] o Comparing effects of light soaking and dark-bias-soaking

Some modules stabilized equally in light vs dark-bias, while others do not o Comparing indoor to outdoor stabilization


Crystalline Si


Light-Induced Degradation (LID) in Crystalline Silicon

Boron-doped crystalline Silicon solar cell material includes: o Czochralski-grown monocrystalline silicon

(Cz-Si) o Cast multicrystalline silicon (mc-Si)

LID in Cz-Si solar cells, Ref. [50]A,[51],[52],[53] o ~4% power output degradation in B-doped

Cz-Si cells during first 5 hours of light soaking o Recovers upon anneal or dark storage [51] o Due to activation of metastable boron-

oxygen defect which lowers carrier lifetime o Greatly reduced using either Ga-doped Cz-Si or low oxygen content B-Cz-Si, [52]

IEC 61215 preconditioning o Qualification for c-Si modules requires 5 hours of preconditioning at 1000 W/m2 prior

to power output measurement

Statistical review of LID in different monocrystalline Cz-Si and mc-Si modules, Ref. [54]

LID may also be present in upgraded metallurgical grade or low-cost silicon, Refs. [55],[56]

Observation of LID in B-doped Cz-Si solar cells.

From Ref. A


References


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